Just thinking aloud here, I'm contemplating a new knife project, not a ballistic knife though which fires off its own blade (painful if you have a jakethebeast<sup>TM</sup> quality blade...) but rather one which incorporates a projectile firing device in the handle.

Something like this beast:

[youtube]http://www.youtube.com/watch?v=wV2s4RvlYcw[/youtube]

... but more compact and definitely not as loud. The design is conceptually identical tothis one.

I calculated 0.4 grams for the delrin pusher piston, at less than 250 fps it should survive the impact at the end of its travel.

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I'd have some sort of vent ahead of the O-ring in extended position, so even if you don't want the propellant volume to leak out, the air in front of the piston wouldn't dampen the initial acceleration too much. But you could still trap some air at the end to cushion the piston impact.

Ohhh pretty! This is pretty much along the same lines of your ballistic knife though, so it should work. The only thing I see going wrong is air compressing past the vent you put in. Unless you can stop the piston just before the vent, its gonna compress the air and possibly mess with the dart's flight. Other than that it looks like it will work.

al-xg wrote:I'd have some sort of vent ahead of the O-ring in extended position, so even if you don't want the propellant volume to leak out, the air in front of the piston wouldn't dampen the initial acceleration too much